The wctrans_t type represents a mapping which can map a wide
character to another wide character. Its nature is implementation-
dependent, but the special value (wctrans_t) 0 denotes an invalid
mapping. Nonzero wctrans_t values can be passed to the towctrans(3)
function to actually perform the wide-character mapping.
The wctrans() function returns a mapping, given by its name. The set
of valid names depends on the LC_CTYPE category of the current
locale, but the following names are valid in all locales.
"tolower" - realizes the tolower(3) mapping
"toupper" - realizes the toupper(3) mapping

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GNU 2015-08-08 WCTRANS(3)